Ignition plug

Abstract

An ignition plug includes a center electrode; a cylindrical insulator that surrounds at least the circumference of a front end portion of the center electrode and that includes a bottom portion at the front side; and a cylindrical metal shell that holds the insulator from the outer circumference side. The center electrode includes a shaft portion that extends along an axial line and a head portion disposed at a front end of the shaft portion. The head portion has a width greater than that of the shaft portion in the radial direction. The insulator includes a first and second insulator. The first insulator has an axial hole and a diameter smaller than the maximum diameter of the head portion. The second insulator is joined to the first insulator. The shaft portion is disposed in the axial hole of the first insulator. The second insulator encloses the head portion.

The invention claimed is: 
     
       1. An ignition plug comprising:
 a rod-shaped center electrode that extends along an axial line from a front side to a rear side; 
 a cylindrical insulator that surrounds at least a circumference of a front end portion of the center electrode and that includes a bottom portion at the front side; and 
 a cylindrical metal shell that holds an outer circumference side of the insulator, 
 wherein the center electrode includes;
 a shaft portion that extends along the axial line, and 
 a head portion that is disposed at a front end of the shaft portion and that has a width greater than that of the shaft portion in a radial direction of the center electrode, and 
 
 wherein the insulator includes;
 a first insulator that has an axial hole in which the shaft portion is disposed and has a diameter smaller than a maximum diameter of the head portion, and 
 a second insulator that is joined to the first insulator and that encloses the head portion. 
 
 
     
     
       2. The ignition plug according to  claim 1 ,
 wherein the first insulator includes a large-diameter portion that protrudes outward in the radial direction, and 
 wherein the metal shell includes a shelf portion with which the large-diameter portion is locked, the shelf portion being provided on the front side of the large-diameter portion and protruding inward further than the large-diameter portion in the radial direction over an entire circumference. 
 
     
     
       3. The ignition plug according to  claim 2 , wherein an outer diameter of at least a portion of the second insulator on the front end of the metal shell is larger than a minimum inner diameter of the shelf portion. 
     
     
       4. The ignition plug according to  claim 1 , wherein the head portion is disposed on the front side of a front end of the metal shell. 
     
     
       5. The ignition plug according to  claim 2 , wherein the head portion is disposed on the front side of a front end of the metal shell. 
     
     
       6. The ignition plug according to  claim 3 , wherein the head portion is disposed on the front side of a front end of the metal shell.